River simulation - different approaches than fluid simulation?
Make a Procedural River using the Ocean Modifier

This technique for creating a river allows the flow of the water to follow the path of the river easily. To do it, simply create an long, narrow Ocean object, then add a Curve Modifier to it.


Create a plane:  SHIFTA -> Mesh -> Plane
(NOTE: For this technique to be easy, DO NOT move the plane away from where it is created -- the 3D Cursor is where it is created)
Add an Ocean Modifier to the plane. You may want to scale down the resulting mesh.  


Lengthen the Ocean object along the X axis by increasing the Repeat X value. This will be the river object:  

 
Add a Path Curve:  SHIFTA -> Curve -> Path
(NOTE: Again, for this technique to be easy, DO NOT move the path away from where it is created -- the 3D Cursor is where it is created, so it will appear right on top of the river)
Scale the path so it is a little bit bigger than the river, then position it so it is in the middle of the river:


Scale: Select path -> S -> drag mouse
Move on X Axis: Select path -> G -> X -> drag mouse
(NOTE: This is the one time you can move the path until you reach Step #10 below)


  
With the path selected, go into Edit mode and subdivide the path as many times as you have bends in the river. In your river, you have one bend, so just subdivide it one time:
Select path -> TAB W -> Subdivide -> Exit Edit Mode (TAB)

 
Select the river and add a Curve Deform Modifier to it and set the Object to the path:  

  

This makes the river mesh conform to the shape of the path.
Now move the river so it is fully on the path:
Select river -> G -> X -> drag mouse

 
Edit the path so it is the shape you want your river to be:  


Position the river on your landscape. To do this, you MUST move both the path and river together, so select them both, then move them:
Select path -> hold SHIFT while selecting the river -> G -> Position them
If you want to make the river narrower or wider in certain places, you can do that by select one of the vertices of the path and altering it with ALTS:
Select path -> TAB -> select vertex -> ALTS -> drag mouse

 


Animating the River

To make the river flow, you now need to animate it's properties by setting keyframes for the Ocean Modifier's properties.